Abstract: A method for producing a liquid phase substance having a pre-established concentration of CO.sub.2 suitable for calibrating analytical instruments such as hemogasanalyzers consists in:preparing an aqueous solution of precisely known strength of a water-soluble salt of carbonic acid,then transferring said solution to a flow reactor containing a cation exchange resin in hydrogenionic form,passing the aforesaid solution of carbonic acid salt through the aforesaid cation resin,thereby achieving the reaction of producing a pre-established quantity of CO.sub.2, andtransferring the eluate thus obtained directly to the analytical instrument for calibration.